For consumer electronic products, it is an important function of digital sound effects, which is outputted on speakers typically by two methods, digital-to-analog converter (DAC) drive method and push-pull drive method such as pulse width modulation (PWM).
FIG. 1 shows the block diagram of a conventional DAC speech synthesizer 100 which includes three basic units, volume control unit 101, signal transform unit 102 and drive unit 103. The volume control unit 101 receives a control signal Vctrl and then generates a control bias Vbias, the signal transform unit 102 receives the control bias Vbias and PCM codes to transform into an analog speech signal Ivo, and the drive unit 103 receives the analog speech signal Ivo and amplifies it to be a current Ispeaker to drive a speaker 104. FIG. 2A is the waveform of a 7-bits sinusoidal PCM signal, FIG. 2B is the waveform of the analog speech signal Ivo after the PCM signal shown in FIG. 2A is processed by the signal transform unit 102 shown in FIG. 1, and the waveform of the output current signal Ispeaker after the analog speech signal Ivo is amplified by the drive unit 103 is shown in FIG. 2C. As shown in FIG. 2C, when a conventional DAC speech synthesizer transforms a digital speech signal back to an analog signal, the current signal Ispeaker has a zero point about 300 mA, which leads to a more power consumption as shown in the area with dashed lines in FIG. 2C. For applications of portable electronic products whose power supply is battery, such large power consumption should be avoided. Moreover, to prevent the transistor 105 within the drive unit 103 from being saturated and resulting in a speech distortion, a bypass resistor 106 is inserted therefor, which further results in speech distortion more seriously.
Shown in FIG. 3 is a push-pull output circuit diagram of a PWM speech synthesizer, which improves the power consumption and needs no additional transistors employed in the speech synthesizer. FIG. 4A is the waveform of a 7-bits sinusoidal PCM signal, FIG. 4B shows the modulated signals SPK1 and SPK2 respectively corresponding to the positive and negative half cycles of the PCM signal shown in FIG. 4A after processed by the push-pull speech synthesizer shown in FIG. 3, and the waveform of the output current signal Ispeaker for the speaker transformed from the PWM modulated signals SPK1 and SPK2 is shown in FIG. 4C.
To drive a PWM speech synthesizer, there is necessary to provide an operation frequencyf=fs×2n−1×m,  [Eq-1]where fs is sampling frequency, n is bit numbers of PCM data, and m is output pulse number for each sample. 2n−1 in Eq-1 represents the resolution of the speech signal. When a desired resolution or output pulse number is increased, the operation frequency is also increased. If more than one sampling frequency are available for a synthesizer, the operation frequency has to be a common multiple of the sampling frequencies and is thus generally high.